The purpose of this Phase III study is to evaluate the efficacy, safety and tolerability and health outcomes of retigabine Immediate Release (IR) as adjunctive therapy to each of the following monotherapy Antiepileptic Drug (AED) treatments: carbamazepine/oxcarbazepine, lamotrigine, levetiracetam, or valproic acid in adult subjects with partial-onset seizures (POS) using a flexible dosing regimen.